Many techniques are known for applying metal coatings to various objects (sometimes called "work pieces" or "substrates").
Within this broad field of metal coating processes, there exists a group of processes in which the metal is evaporated or vapourized and then deposited on the object to be coated, whether by condensation, ionic attraction, or otherwise.
In one type of vapour deposition coating system the metal vapour is formed as a metal plasma from the surface of an electrode that is positioned within suitable coating apparatus. Such systems are shown in, for example, U.S. Pat. No. 2,972,695 and U.S. Pat. No. 3,793,179.
In vapour deposition coating systems in which the plasma of coating material is generaed from a sacrificial electrode, the plasma is often initiated by a trigger or secondary electrode which momentarily serves to generate an electric arc which creates a spot or series of spots on the primary electrode (e.g. cathode) from which the plasma is generated. After the plasma is initiated, the trigger electrode is silenced and current flow is between the anode and cathode. As the arc current flows between the anode and cathode, plasma-generating spots are created on the sacrificial electrode and the spots tend to move randomly or chaotically across the surface of the sacrificial electrode.
In operating vapour deposition apparatus of this type, the sacrificial electrode is often in the form of a disc and the electrode can be used repeatedly until the metal of the electrode has been consumed. However, from a practical standpoint it has been found that as plasma is generated from the sacrificial electrode, the electrode is not eroded in a uniform way. As a result, the sacrificial electrode must be replaced periodically even though a substantial percentage of its original weight may still exist.